1. Field of the Invention
The present invention relates to an apparatus for post-processing sheets which is installed at a sheet discharge end of a longitudinal discharge type image forming apparatus, such as a duplicator, a printer, and a facsimile, in which the sheets are transferred in a longitudinal direction (a lengthwise direction of the sheets), arranges in a temporary storage space the longitudinal sheets on which images have been formed, and performs post-processing such as stapling of the arranged sheets, and more particularly to an apparatus for post-processing sheets which performs an stapling operation in a transfer direction in a process of transferring a sheet stack arranged in a temporary storage space.
2. Description of the Prior Art
In general, an image forming apparatus has one or two or more of a copy function, a print function, and a fax function according to a recent development from an analogue method to a digital method, and a digital type image forming apparatus can selectively discharge image-formed sheets from the apparatus itself.
An analogue type image forming apparatus of the related art discharges the sheets whenever a required number of documents are sequentially fed into the apparatus through an automatic document feeder over and over again, and multiple sheet receiving trays installed in a sheet post-processing apparatus for arranging the sheets differentially receive the discharged sheets while moving upward and downward.
However, the digital type image forming apparatus stores image information to be copied or recorded in a main body, and then records the image information on sheets in an order desired by a user to discharge the sheets. Accordingly, a sheet post-processing apparatus having a structure in which the sheets are differentiated and received without the multiple sheet receiving trays is being proposed.
For example, when a user wants to copy or record a multipage document many times, the main body of the image forming apparatus records image information of the multipage document on sheets and discharges the sheets in a serial order over and over again, and such a new sheet post-processing apparatus, including a sheet receiving tray for temporarily receiving the sheets until a stack of sheets is completely discharged, performs post-processing such as stapling of the sheets if necessary when the stack of sheets is completely discharged and then stores the sheets in one tray having a stacker function.
At this time, the sheet receiving tray for temporarily receiving the sheets requires a function of the post-processing such as stapling to be performed if necessary, when the stack of sheets is completely discharged or a sheet arrangement function for allowing the sheets to be received in an arranged state, and the one tray having the stacker function requires an offset function for differentially storing sheet stacks in zigzags such that the post-processed sheet stack is stored differentially from a next discharged sheet stack.
Such a new sheet post-processing apparatus has been proposed in Japanese Patent No. H08-941 and Japanese Patent No. 2583594, and will be described with reference to FIGS. 1 and 2.
As shown in FIG. 1, a sheet post-processing apparatus is mounted to a sheet discharge hole 2 side of an image forming apparatus 1 such as a duplicator, a facsimile, and a printer. An ejected sheet on which an image has been formed is discharged to a sheet discharge hole 9 of the sheet post-processing apparatus through a sheet transfer unit 4. First, the discharged sheet is received in an inclined arrangement tray 5, and an inner side end of the sheet is supported by a sheet push unit 8 located on an inclined lower surface of the arrangement tray 5 to be arranged. At this time, the sheet received in the arrangement tray 5 is moved toward the inclined lower surface through a conveying belt 7 such that the inner side end of the sheet is more accurately arranged. When a stack of sheets is completely discharged through repetition of the above-described process, a solenoid 5a connected to the arrangement tray 5 operates to arrange a side surface of the received sheet stack, and post-processing of the sheets is performed through an operation of a stapler 10 in a state in which the sheets have been arranged. Thereafter, the sheet push unit 8 supporting the lower surfaces of the sheets is moved forward by a sheet push motor 8a so that the sheets are received in a receiving tray 6.
As shown in FIG. 3, in the sheet post-processing apparatus of the related art, a stapler 10 is generally installed at a leading end portion or a trailing end portion of the sheets in a discharge direction, and performs stapling while crossing a discharge path of the sheets. FIG. 3A shows a state in which a stapler 10 performs stapling at a trailing end portion of longitudinally transferred sheets (sheets in which a lengthwise direction corresponds to a transfer direction) while crossing a discharge path of the sheets, and FIG. 3B shows a state in which a stapler 10 performs stapling at a trailing end portion of laterally transferred sheets (sheets in which a lengthwise direction corresponds to a direction perpendicular to a transfer direction) while crossing a discharge path of the sheets.
FIG. 3A and FIG. 3B show examples of multipoint stapling in the sheet post-processing apparatus mounted to a main body capable of transferring large-sized sheets, for example, A3 size sheets or double-letter size sheets.
However, in a case of a main body capable of transferring only small-sized sheets, for example, A4 size sheets or sheets having a size equal to or smaller than letter size, the sheets can be transferred only in a longitudinal direction (a lengthwise direction of the sheets), and a sheet post-processing apparatus mounted to the main body can perform only one-point stapling at a leading end portion or a trailing end portion of the sheets as shown in FIG. 3A.
Accordingly, the sheet post-processing apparatus mounted to the main body capable of transferring the small-sized sheets only in the longitudinal direction cannot bind the sheet stack like a book.